A universal joint 100 is for connecting two shafts, the axes of which cross each other at a certain angle, and transmitting a rotational force of the one shaft to the other shaft. For example, as shown in FIG. 9, the universal joint 100 is configured with: a cross shaft 101, the axes of which are perpendicular to each other; and two yokes 102 and 105 (a first yoke 102 and a second yoke 105) which are each connected to both end portions of each of the shafts configuring the cross shaft 101 and arranged pivotably about these axes, and to which one end sides of rotating shafts 111 and 112 are connected, respectively.
Such a universal joint 100 is provided on a steering device for an automobile or the like, for example. This steering device is configured to give a steering angle to front wheels by transmitting a rotational force of a steering shaft 111 which is rotated by operating a steering wheel to an input shaft 112 of a steering gear via the universal joint 100.
The first yoke 102 comprises: a base portion 103; and a pair of opposed walls 104 which is extended from the base portion 103 so as to face each other at a distance. The one end side of the steering shaft 111 is fixedly provided on the outer surface of the base portion 103, and the cross shaft 101 is mounted to the inner surfaces of the distal end side of the opposed walls 104.
The second yoke 105, as shown in FIG. 9 to FIG. 11, comprises: an insertion portion 106 formed in a cylindrical shape having an opening 106a which opens along the axial direction of the input shaft 112 of the connected steering gear, the insertion portion 106, the inner circumferential surface of which comes into contact with the outer circumferential surface of the input shaft 112 when the one end side of the input shaft 112 is inserted thereinto, and; a pair of tightening portions 107 which is extended from the side of the joint with the insertion portion 106 to the opposite side thereof so as to face each other with the opening 106a of the insertion portion 106 between them; a pair of opposed walls 108 which is extended toward the opposite side to the connection with the input shaft 112 and along the axial direction of the input shaft 112, and which is arranged to face each other at a distance; and a bolt 109 for tightening the pair of tightening portions 107.
A serration 106b which can engage with a serration 112a formed on the outer circumferential surface of the one end side of the input shaft 112 is formed on the inner circumferential surface of the insertion portion 106. A through hole 107a is formed in one of the pair of tightening portions 107, and a screw hole 107b is formed in the other co-axially with the though hole 107a. The bolt 109 is inserted into the through hole 107a and screwed into the screw hole 107b. A mounting hole 108a in which the cross shaft 101 is mounted is co-axially formed in each of the distal end portion of the pair of opposed walls 108.
According to this second yoke 105, after the one end side of the input shaft 112 is inserted into the insertion portion 106, when the pair of tightening portions 107 is tightened by the bolt 109 which is inserted into the through hole 107a and screwed into the screw hole 107b, the input shaft 112 is held and fixed by the insertion portion 106. Thereby, the second yoke 105 and the input shaft 112 are connected and fixed.
At the one end side of the input shaft 112, an annular groove 112b is formed. By the engagement between the groove 112b and the outer circumferential portion of the bolt 109, the movement of the input shaft 112 in the axial direction thereof is regulated. Thereby, the input shaft 112 is prevented from coming out of the second yoke 105.
In the above described second yoke 105, when the pair of tightening portions 107 is tightened by the bolt 109, as shown in FIG. 11, the distal end sides of the tightening portions 107 are deformed so that they approach each other and the bolt 109 bend. Therefore, not only tensile stress but also bending stress acts on the bolt 109, thereby, causing a problem that fatigue strength (durability) of the bolt 109 is decreased. That is, when a force by the rotational force transmitted via the universal joint 100 acts on the bolt 109 repeatedly in a state where the tensile stress and the bending stress act thereon, the time until the bolt is broken with fatigue is remarkably shortened.
As a yoke (second yoke 105) which appears to be able to remove such convenience, conventionally, there is the yoke disclosed in Japanese Unexamined Patent Application Publication No. 7-317793, for example. Similarly to the second yoke 105, this yoke 120 comprises: the insertion portion 106; the pair of tightening portions 107; the pair of opposed walls 108; and a bolt 109 and nut 121 for tightening the pair of tightening portions 107. The same components as the second yoke 105 are assigned the same reference numerals, and detailed explanations thereof are omitted.
On the other of the pair of tightening portions 107, a through hole 107c is formed instead of the screw hole 107b. The pair of tightening portions 107 is tightened by the bolt 109 and the nut 121 which are inserted into the through holes 107a and 107c. Thereby, the input shaft 112 is held and fixed by the insertion portion 106.
At distal end of the pair of tightening portions 107, contact portions 107d which are extended toward the facing direction of the tightening portions 107, and which can come into contact with each other. When tightened by the bolt 109 and nut 121, these contact portions 107d come into contact with each other. Thereby, the distal end sides of the tightening portions 107 are prevented from approaching each other.    Patent document 1: Japanese Unexamined Patent Application Publication No. 7-317793.